what you want to hear. 30 years since last paint job and maybe that many years of nic buildup.... you need to remove all that stuff first. that is like brown ooze tar and belive me, nothing will stick to it and not too many things will cover that and allow for a nice paint job. Get some soap and water, hot water and clean that stuff off first. Be carefull about too much water on that wall and rock, and then after cleaning, wait a couple of days before painting, it has to thoroughly dry.